1. Field of the Invention
The present invention generally relates to power control for electrical devices, and more particularly to an overvoltage protection circuit particularly useful in overdriving light sources such as a lamp for an overhead projector.
2. Description of the Prior Art
Overhead projectors (OHP's) are known in the art, and generally include a base having a light source, a transparent stage above the light where an image-bearing transparency is laid, and a projection head which collects light passing through the transparency and projects in on a wall or screen. It is desirable to provide a high-intensity light source to ensure that the projected image is sharp and the colors vivid; this also allows more light to be present in the room, for the audience to read other materials. Light intensity has become even more important with the use of liquid crystal display (LCD) panels in conjunction with OHP's, since these panels use polarizers which greatly diminish the intensity of the light passing therethrough.
For a given OHP system, there are basically two methods of improving overall brightness of the projected image, namely, increasing the amount of light from the light source, or collecting the light more efficiently toward the stage. With respect to the former method, there are several techniques which allow for the production of more light at the light source. These include (i) providing multiple light sources whose output is combined, (ii) using different types of light sources, such as halogen or arc lamps, and (iii) overdriving the lamp (i.e., exceeding the nominal manufacturer's rated voltage). Of course, these approaches may all be used together. If the latter of these techniques is used, however, it presents a greater risk of lamp failure since a minor fluctuation in the power source (a transient increase in the ac voltage), can cause the lamp to be overdriven too much.
For example, a conventional EVD incandescent halogen lamp has a maximum manufacturer's voltage rating of 36 volts, and a "kill" voltage of about 40 volts (i.e., a 40 volt surge will burn out the lamp's filament). With a typical external electrical ac source (120 volts rms) the amplitude may deviate .+-.10% within accepted limits, so a conventional power supply/regulator in an OHP might generate a lamp voltage of up to 39.6 volts, pushing the limits of the lamp, but still under the kill voltage. If, however, the lamp is already overdriven, say at 38 volts, then such a surge would push the lamp voltage above the kill voltage, in all likelihood burning out the filament. Although overdriving the lamp can shorten its life, this may certainly be an acceptable compromise for increased luminosity, as long as the lamp could survive the transient surges.
While there are a variety of conventional overvoltage protection methods, prior art projection systems lack any overvoltage protection for overdriven light sources. Indeed, there presently are no such commercially available overdriven OHP's, due in part to the difficulty of providing proper overvoltage protection. The lack of such protection is due to the inability (or commercial unfeasibility) to use existing circuits in a system wherein an ac voltage is to be regulated, and in such a manner that allows overdriving of the voltage. For example, many prior art protection circuits are limited to regulation of dc output voltage. U.S. Pat. No. 4,815,052 describes an automatic overvoltage protection circuit for a system wherein a dc power supply is generated from a variable-speed three-phase alternator. The system uses three sets of silicon-controlled rectifiers (SCR's) to adjust firing signals for a gated, three-phase rectifier bridge. U.S. Pat. No. 4,916,085 depicts integrated overvoltage protection in a metal-oxide semiconducting (MOS) power structure. The emitter, collector and base of a conventional bipolar transistor are connected, respectively, to the gate, drain and source of the MOS cell; a zener diode may additionally used as an overvoltage blocking diode;. U.S. Pat. No. 4,914,540 discusses overvoltage protection for a switching element wherein a MOSFET has a parasitic capacitance providing high frequency shorting across the switch. In U.S. Pat. No. 5,122,726, protection is provided to a bank of redundant, parallel power supplies, by removing any power supply which has failed, including failures wherein the output bus (dc) is driven to an overvoltage condition. U.S. Pat. No. 5,130,883 discloses another invention relating to overvoltage protection for semiconducting devices, such as fieldeffect transistors (FET's). In that construction, the circuit switches from the input line to a capacitor which maintains the bias on the gate of the FET without exposing it to the surge. The apparatus of U.S. Pat. No. 5,225,958 uses a combination of relays and diodes to alleviate excess voltage on CCD elements. U.S. Pat. No. 5,233,497 illustrates a circuit which achieves overvoltage protection by using zener diodes and an SCR (or thyristor) to clip low-energy overvoltage pulses and short high-energy pulses; see also U.S. Pat. Nos. 5,243,488, 5,245,499 and 5,243,205. A parasitic SCR equivalent is shown in U.S. Pat. No. 5,235,489, which provides a controllable breakover voltage for the supply line voltages on a semiconductor substrate. None of the foregoing designs are readily adapted to powering an ac lamp. It would, therefore, be desirable and advantageous to devise a power supply for an OHP lamp which allows overdriving of the lamp, yet still protects against overvoltages which might, in combination with overdriving, push the lamp past its kill voltage.